Electrospray is a method of generating a very fine liquid aerosol through electrostatic charging. Electrospray, as the name implies, uses electricity to form small droplets. In electrospray, a plume of droplets is generated by electrically charging a liquid passing through a nozzle to a very high voltage. The charged liquid in the nozzle is forced to hold more and more charge until the liquid reaches a critical point at which it ruptures into a cloud of tiny, highly charged droplets.
Electrospray is referred to as “electrospray ionization” (ESI) when used as an ionization method for chemical analysis. ESI is the process of generating a gas phase ion from a typically dissolved solid or liquid chemical species. The electrospray process allows the structural analysis of unlimited molecular weight, e.g., large biomolecules, in the field of mass spectrometry and is directly compatible with liquid chromatography methods. Ionization is an important event in mass spectrometry by allowing accurate mass to charge ratio measurements of ions. A mass spectrometer is an instrument which can measure the masses and relative concentrations of atoms and molecules by evaluating a number of forces on a moving charged particle. Once an ion's mass is ascertained, this information can be used to determine its chemical composition.
U.S. Pat. No. 6,949,742, Figueroa, entitled “Method and A System for Producing Electrospray Ions” discloses a number of prior art electrospray configurations. FIG. 1 illustrates one such prior art electrospray configuration. The electrospray ion source 2 configuration includes a gas source 4 such as compressed nitrogen and a sample material source 6 being fed directly to a plurality of platinum concentric needles 8. The gas source 4 forces a constant quantity per unit time of the sample material through the platinum concentric needles 8 producing a continuous flow of sample spray or aerosol 10. A potential is then generated on a counter electrode 12 by a power supply 14 on the aerosol 10 causing a continuous flow of electrospray ions 16 to be directed to a number of Einzel/ion lenses 18 and subsequently to a mass analyzer 20.
This configuration suffers a number of disadvantages. The use of electrostatics on the aerosol to obtain ions is effective only for solvents and solvent mixtures having certain properties. It may not be as effective when used on solvents and solvent mixtures having other properties. The aerosol 10 formed is also not focused at an inlet of the mass analyzer 20 but tends to cover a wide area around the inlet. This spread of the aerosol results in the mass analyzer 20 receiving only a portion of the charged ions in an area immediately adjacent the inlet. Consequently, the aerosol 10 is not evenly sampled and may result in limited sensitivity of the mass spectrometer. This spread of the aerosol 10 may also result in differences in the rate at which charged ions from different areas of the aerosol arrive at the inlet, which may lead to band broadening.
FIG. 2 illustrates the components of a prior art thermal inkjet (TIJ) electrospray ion source 30 disclosed in the patent for solving the abovementioned problems. The thermal inkjet electrospray ion source 30 includes a sample source 6 or sample reservoir fluidly coupled to a thermal inkjet material dispenser 32. Additionally, a computing device 34 may be communicatively coupled to the thermal inkjet material dispenser 32 according to one exemplary embodiment. An electrically conducting grid 34 is disposed adjacent to the thermal inkjet material dispenser 32 in the path of the nozzles of the thermal inkjet material dispenser 32. A counter electrode 12 coupled to a plurality of Einzel/ion lenses 18 that lead to a time-of-flight mass analyzer 36 are disposed opposite the electrically conducting grid 34. Both the electrically conducting grid 34 and the counter electrode 12 are electrically coupled to a power supply 14 configured to independently vary the voltage at the electrically conducting grid 34 and the counter electrode 12. As can be seen in FIG. 2, the thermal inkjet electrospray ion source 30 allows for a linear configuration while providing a pulsed material sample to the mass analyzer 36.
The thermal inkjet electrospray ion source 30 illustrated in FIG. 2 is configured to generate small droplets of a sample material using the thermal inkjet material dispenser 32. These generated droplets of sample material then react to a potential generated between the conducting grid 34 and the counter electrode 12. In response to the generated potential, the droplets of sample material are accelerated towards the Einzel/ion lenses 18 and the mass analyzer 36. During this acceleration, an electrospray process occurs and the charged ions of the sample material are formed. In further detail, the electrospray process begins with an accumulation of positively charged ions in the small droplets of sample material, causing surface instability. When the Coulombic repulsions, or the repulsion among similarly-charged regions of a particle, between the positively charged ions exceed the surface tension of the sample material, smaller droplets will start to come off the surface of the liquid, forming a mist. As these droplets travel towards the counter electrode 12, a solvent portion of the sample material evaporates causing the droplets to shrink and, as a consequence, the distance between positive charges at the surface of the droplets become smaller and charge repulsion gets stronger. This process continues until the Coulombic repulsions are stronger than the surface tension of the droplet (a condition called the Rayleigh instability limit) causing the droplet to explode into smaller charged droplets of analyte molecules ready to be analyzed in the mass analyzer 36.
Although this thermal inkjet electrospray ion source overcomes some of the abovementioned disadvantages associated with the electrospray ion source in FIG. 1, there remain disadvantages associated with the above described ESI techniques where the aerosol is subjected to a generated electric potential such that charge is transferred to the analyte via the solvent. In other words, the solvent is charged in order to charge the analyte. The mass analyzer can receive only a certain amount of charged ions. However at an inlet of the mass analyzer, not all solvent is evaporated. Both analyte and solvent ions are thus received by the mass analyzer. With the receiving of both the charged solvent ions and the charged analyte ions, the sensitivity of the mass analyzer may be reduced. Furthermore, the charged solvent ions may also interact with the charged analyte ions to result in ion suppression and unreliable quantization. The dynamic range of the mass analyzer is limited by the amount of charge that can be placed on the liquid. Moreover, the ESI technique will not ionize all compounds. ESI discriminates against very non-polar molecules. ESI is also susceptible to adduct formation that complicates spectral interpretation and creates non-linearities when trying to generate a linear calibration curve. It would thus be desirable to have an ion source that overcomes at least one of the above remaining disadvantages.